Concrete and other masonry or cementitious materials have high compressive strength, but relatively low tensile strength. When concrete is employed as a structural member, such as in a building, bridge, pipe, pier, culvert, or the like, it is conventional to incorporate reinforcing members to enhance the tensile strength of the structure. Historically, the reinforcing members are steel or other metal reinforcing rods or bars, i.e., "rebar". Such reinforcing members may be placed under tension to form prestressed concrete structures.
Although steel and other metals can enhance the tensile strength of a concrete structure, they are susceptible to oxidation. For example, ferrous metal rusts by the oxidation thereof to the corresponding oxides and hydroxides of iron by atmospheric oxygen in the presence of water. When it is poured, concrete is normally at a pH of 12 to 14 (i.e., at high alkalinity) due to the presence of hydroxides of sodium, potassium, and calcium formed during the hydration of the concrete. As long as a pH in this range is maintained, steel within the concrete is passive, which may result in long-term stability and corrosion resistance.
Exposure to a strong acid, or otherwise lowering the pH of concrete, can cause steel contained in concrete to become corroded. For example, chlorine ions permeating into the concrete can cause corrosion. Sources of chlorine ions include road salt, salt air in marine environments, and salt-contaminated aggregate (e.g., sand) used in making the concrete. When the reinforcing steel corrodes, it can expand and create internal stresses in the concrete. These internal stresses can lead to cracking, and ultimately disintegration, of the concrete. Moreover, cracking and crumbling concrete may expose additional steel to atmospheric oxygen, water, and sources of chlorine ions.
Various solutions to the corrosion problem of steel rebar have been offered. Non-corrosive coatings on the concrete, the steel rebar, or both have been proposed. For example, U.S. Pat. No. 5,271,193 to Olsen et al. proposes a steel-reinforced concrete product, such as a manhole cover, having a coating of a corrosion-resistant gel coat layer and an intermediate layer of fiberglass between the concrete and the gel coat layer. The gel coat layer is described as being a "hardenable polymeric fluid material." U.S. Pat. No. 4,725,491 to Goldfein proposes steel rebar members having chemical conversion iron oxide coatings, such as black iron oxide. U.S. Pat. No. 5,100,738 to Graf proposes steel rebar having an outer layer of a synthetic material (e.g., epoxy resin) and an intermediate layer of aluminum or aluminum alloy between the outer layer and the steel. Unfortunately, in general these exemplary coatings tend to be expensive and have received mixed results and acceptance.
There has also been interest in replacing steel with various fiber-reinforced resins. For example, U.S. Pat. No. 5,580,642 to Okamoto et al. proposes a reinforcing member for civil and architectural structures made from a mixture of reinforcing fibers and thermoplastic fibers. U.S. Pat. No. 5,613,334 to Petrina proposes a non-metallic laminated composite reinforcing rod for use in reinforced or prestressed concrete. A corrosion-resistant fiber-reinforced rebar, disclosed in U.S. Pat. No. 5,650,109 to Kaiser et al. comprises a fiber reinforced thermoset core and an outer cladding formed of sheet molding compound (SMC). These materials are formed into rebar through modified pultrusion processes.
Some rebar components are desirably curved or bent in order to follow the contour of the surrounding concrete structures. Unfortunately, rebar formed from fiber-reinforced resins may be difficult to bend in the field without causing the rebar to crack or break. Forming non-linear articles via pultrusion processes may also be troublesome. Because pultrusion involves pulling material through an elongated heated die which at least partially cures, and therefore stiffens, the pultruded article, establishing bends or curves-in the articles without sacrificing the advantages provided by pultrusion may be problematic.